1. Field of the Invention
The present invention relates 1,4-bis(2'-isopropyl-6'-methylanilino)anthraquinone to a polyethylene terephthalate film containing this anthraquinone compound for use as an X-ray photographic film support and, more particularly, to a blue-colored polyethylene terephthalate film resulting from the use of this anthraquinone and having particular utility as an X-ray photographic film support.
2. Discription of the Prior Art
In X-ray photographic films, it is desirable to color the support film blue in order to facilitate the discrimination of the photographic images. Also, it is desirable for the film support not to absorb light in the short wavelength region since an X-ray photographic film has a light-sensitive region in the short wave-length region. Such a support serves to prevent the disadvantages with photographic materials in which a photographic emulsion is provided on both surface of the film support, such as X-ray photographic films, that a filter desensitization action results upon photographing and that photographic images having yellow fog and being difficult to discriminate result.
A dye for coloring polyethylene terephthalatefilms for use as an X-ray photographic film support to achieve the above-described objects must satisfy the requirements of heat resistance, sublimation resistance, compatibility and suitable chromaticity at the same time, must be inert with respect to the gelatin-silver halide emulsion, and must not exert detrimental influences on the photographic properties such as sensitivity, gamma, fog, etc.
Polyethylene terephthalate films are industrially produced using a heat melt extrusion methodh. Therefore, the coloring agent must have sufficient heat resistance to withstand elevated temperatures as high as 270.degree. to 300.degree.C.
Since polyethylene terephthalate film has extremely stable physical structure, coloring methods such as dyeing are not easy. Therefore, a dope-dyeing method is preferable in which a coloring agent is mixed with the polymer either upon synthesis of the polymer or upon heat-melt molding of the polymer to thereby disperse or dissolve the coloring agent in the polyethylene terephthalate. Therefore, the coloring agent must withstand molding temperatures as high as 270.degree.- 300.degree.C and to possess sufficient heat stability not to be decomposed or faded by heat.
The dope-dyeing method which is as described hereinafter employed in the present invention has the advantage that coloring is conducted simultaneously with molding and no additional steps are required.
Polyethylene terephthalate films are prepared by drying the starting pellets, mixing the pellets with a coloring agent and, after heat melting the mixture, extruding, stretching and heat-treating the melt, and forming it into film. Of these steps, the drying step is desirably conducted at temperatures of from 135.degree. to 210.degree.C under reduced pressure as described in Japanese Pat. Publication No. 618/53. Without the drying step, the starting polyester would undergo hydrolysis upon heat-melting, which results in it being impossible to make the film or in producing a film with seriously deteriorated properties, e.g., having low strength.
On the other hand, since polyester films are usually formed into a film according to a tenter method, edge loss is unavoidable. In order to recover and re-use this edge waste, it is crushed and dried together with virgin chips. When the polyester is colored with a coloring agent having poor sublimation resistance, the dye will be sublimed away from the colored edge waste during the drying. Therefore, in addition to the loss of the coloring agent, the sublimed dye stains the dryer and drops falling therefrom on an irregular basis cause unevenness in color density. Also, polyethylene terephthalate is extruded through an extrusion die onto a casting drum to cool. At this time, when the polyester is colored with a easily sublimable coloring agent, the sublimed coloring agent stains the casting drum causing unevenness in dye density of the colored film itself. For these reasons, it is important to color the polyester with a coloring agent having good sublimation resistance.
In conducting the dope-dyeing, it is necessary for the coloring dye to be easily dispersed or dissolved uniformly in polyethylene terephthalate. In particular, for photographic film supports, high transparency and optically defect-free surface conditions are required. Therefore, insufficient dispersion of the coloring agent, rendering the support cloudy, and contamination with granular foreign substances cannot be allowed in the slightest.
As is well known, in order to provide polyethylene terephthalate with useful properties, melt-filmed melt-extruded, melt cast, melt quenched etc. (hereinafter melt-filmed), amorphous, non-oriented polyethylene terephthalate film must be subjected to stretching and heat-treatment under suitable temperature conditions. Therefore, even when the coloring agent is apparently dispersed uniformly in an amorphous, non-oriented film to such an extent that the transparency is not deteriorated, often in the subsequent stretching step, spaces between the fine, dispersed particles of the coloring agent incapable of being plastically deformed and polyethylene terephthalate matrix capable of being plastically deformed to a great extent arise, these vacancies scatter light increasing the cloudiness of the film. Accordingly, in the production of, particularly, photographic film bases, sufficient care is required in dispersing the coloring agent as extremely fine particles so as not to produce spaces in the stretching step which substantially increase the light scattering, or to dissolve the coloring agent in the form of a molecular dispersion.
An object of the present invention is to provide a dye which is heat stable and which can be used in particular with polyethylene terephthalate as a blue dye for an X-ray photographic film support.
It is further an object of this invention to provide a dye which has appropriate blue color characteristics permitting its use as a dye for an X-ray photographic film support and which does not adversely affect the photographic characteristics of the emulsion when it is used in a photographic support.
It is additionally an object of this invention to provide a coloring agent which can be used in a photographic film support and not give rise to a decrease in transparency nor to a cloudiness.